The present invention relates to lasers with unstable resonator cavities; and more particularly, the invention relates to line selection of and in such lasers.
It is inherent in many laser media that they are capable of lasing at different frequencies. Moreover, the type of stimulation involved may result in multiple line lasing and resonating in the optical laser cavity. This is particularly true in chemical lasers in which the chemical reaction produces population inversions resulting in different transitions, i.e., emission of different frequencies. In many instances, multifrequency emissions are not wanted for a variety of reasons. Among them, for example, is the possibility of absorption of the most strongly developed lines by the environment through which the emitted laser beam propagates.
Frequency selectivity of lasers in general and of unstable resonator cavities in particular has been dealt with in the past. See, for example, U.S. Pat. Nos. 4,123,149 and 3,928,817. These prior art devices have in common that the selectivity to be effective must cover the entire width of the laser beam, and uniformly so in order to be effective. This requirement poses considerable problems for the uniformity of the element or device which preforms the selection, and there are other practical limitations with large devices, particularly for lasers in which the lasing medium has a relatively large volume.